Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, and the like. However, among them, a lithium ion secondary battery involves a safety issue such as a fire or explosion due to the use of an organic electrolyte solution, and has a disadvantage of being complicated to manufacture. A lithium polymer secondary battery designed to overcome these weak points of a lithium ion secondary battery is stated to be one of the next-generation batteries of the future, but still has a relatively low capacity, in particular, an insufficient discharging capacity at low temperature, when compared to a lithium ion secondary battery, and accordingly, there is an urgent demand for improvement.
For this, there is a growing need for a high capacity anode material, and a (quasi)metal material having a high theoretical capacity, for example, Si— and Sn-based materials, is being applied as an anode active material, however these anode active materials deteriorate in cycle characteristics as charging and discharging repeats, and their extreme volume expansion has a negative influence on the performance and safety of a battery. Accordingly, studies have been conducted to improve the cycle characteristics and mitigate the volume expansion by using (quasi)metal oxide, for example, silicon oxide (SiOx) and the like, however (quasi)metal oxide has a shortcoming of significantly low initial efficiency because (quasi)metal oxide produces an irreversible phase due to an initial reaction of oxygen and lithium upon lithium insertion.
To compensate this defect, if (quasi)metal oxide is prealloyed with lithium so that (quasi)metal oxide contains lithium, an irreversible phase such as lithium oxide or lithium metal oxide is produced in a reduced amount during initial charging/discharging of a battery, resulting in increased initial efficiency of an anode active material.
A lithium source for pre-alloying (quasi)metal oxide with lithium may be largely classified into a lithium metal, a lithium salt free of oxygen, and a lithium salt containing oxygen.
Among them, a lithium salt free of oxygen is mostly ionically bonded, and thus its reaction with (quasi)metal oxide is extremely limited. Also, the use of a lithium salt containing oxygen hinders the adjustment of an oxygen content of (quasi)metal oxide since oxygen in a lithium salt reacts with (quasi)metal oxide in the process of a reaction between (quasi)metal oxide and a lithium salt containing oxygen. Also, due to a by-product derived from a reaction between an un-reacted lithium source residue and (quasi)metal oxide, there is still a gelation issue of an anode active material slurry in a battery fabrication process.
Meanwhile, in case a lithium metal is used as a lithium source, there are disadvantages such as high reactivity with water and high fire hazards, and formation of lithium carbonate caused by reaction with carbon dioxide.